This invention relates to an electronic printer which uses an addressable image writing array (write bar) to form images on a moving photoreceptor surface. More particularly, the invention is directed to a print system utilizing a first image bar for high resolution printing in cooperation with a second image bar for low resolution exposure.
Prior art printing systems which utilize linear image bars, composed of a plurality of, for example, individually activated lightemitting diodes (LEDs) are known in the art. The images are formed on a photoreceptive surface by selectively energizing each LED in response to input binary digital image signals. Representative print systems are disclosed, for example, in U.S. Pat. Nos. 4,801,978; 4,689,694; and 4,731,673.
The image bar printer creates an image-wise discharge of a charged photoreceptive surface in one of two operational modes. The first is characterized as a discharge area development, sometimes known as a "write black" mode in which the individual LEDs are activated (turned on) to discharge (expose) a portion of the photoreceptive surface corresponding to black (informational) areas of the image being printed. The complete latent image thus consists of discharged areas, corresponding to the image, and undischarged areas corresponding to (usually white) background. The subsequent conventional prior art step of development is adapted to attract toner to the discharged areas to form a developed image suitable for transfer to a recording medium.
The second operational mode is a charged area development, sometimes known as a "write white" mode in which individual LEDs are activated to expose the background, leaving the informational (image) areas in the charged state. The subsequent development step is thus adapted to attract toner to the still charged areas to form the developed image.
Documents to be printed have, on average, about 7-10% text area coverage. Thus, an LED image bar operated in a charged area development write white mode has a considerably higher duty cycle than an image bar operated in the discharged area development write black mode; e.g., the write white bar will be "on" 90% of the time while the write black bar will only be "on" 10% of the time. This higher duty cycle shortens the life of the write white bar. In addition to the problem of shorter life, the write white bar requires higher maximum current and generates more heat, which may require some form of cooling. Thus, a write black system would appear to be the preferred system for most, but not all, conventional LED image bar printers. However, there are additional considerations involving certain trade-offs which might make the use of a write white system attractive. One factor is a requirement that a particular printing system be compatible with an already existing light lens copying system. Virtually all light lens conventional copiers direct light reflected from a scanned original (light lens) to the photoreceptive surface. The discharged areas are then the background areas and the charged areas represent the information areas. In other words, the conventional light lens copier operates in a write white mode. The photoreceptor and development system are selected to be compatible with operation in this mode. If it is desired to add a high resolution LED image bar, to a light lens copier to enable both copier and printer functions, or if it is desired to enable a printer using a light lens xerographic engine as the base engine, the image bar will have to operate in the write white mode.
According to a first aspect of the present invention, the prior art write white system is modified by including a second, low resolution image bar whose function is to discharge areas of the photoreceptor corresponding to selected, non-informational background areas of the image. This greatly reduces the use rate of the higher resolution bar which will only be used to expose the informational areas. Thus, the life of the high resolution image bar is extended. Significant cost saving are achieved by less frequent replacement of the more expensive high resolution bar. The low resolution bar is much less expensive than the high resolution bar and, due to higher operating tolerances, has an operating life which is greater than the life of the high resolution bar. According to a second aspect of the invention, the low resolution print bar can also be used to provide other functions such as inter-document erase, and, patch generation for diagnostic test purposes.
It is known to use multiple LED image bars in the printing system, but in a different configuration, and for different purposes. U.S. Pat. No. 4,926,200 discloses a printer having two LED image bars of different resolutions, one for text and a second bar for pictorials. Thus, both LED image bars are simultaneously energized for printing information, albeit of different resolutions.
More particularly, the present invention relates to an image bar printing apparatus for exposing the surface of a photosensitive medium, including in combination a first, high resolution image bar, a second, low resolution image bar, and input data control means for selectively energizing said first and second bars so as to expose a first selected area of said photosensitive medium with said first image bar, and to expose a second selected background area of said photosensitive medium with said second image bar.